The administration of Interleukin-2 to patients with neoplasms is associated with the significant incidence of lymphocytic myocarditis, myocardial necrosis, and a vascular leak syndrome. In this project, a small animal model was developed to study these phenomena. The administration of IL-2 to rats produced lymphocytic myocarditis and myocardial necrosis with evidence of damage to the microvasculature and the myocytes by lymphokine-activated lymphocytes.